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Abstract:

A multimeter includes a main body, two probes extending from the main
body, a battery unit arranged in the main body, and a charging system
arranged in the main body and configured for charging the battery. The
charging system includes a microcontroller with an external input voltage
sampling circuit, a battery voltage sampling circuit and a voltage
regulator circuit each electrically connected the microcontroller. The
microcontroller compares sampled signals from the external input voltage
sampling circuit and the battery voltage sampling circuit, and controls
the voltage regulator circuit to regulate the external input voltage to
be applicable to the battery based on the comparison.

Claims:

1. A multimeter, comprising: a main body; two probes extending from the
main body; a battery unit arranged in the main body; and a charging
system arranged in the main body and configured for charging the battery
unit, the charging system comprising: a microcontroller; an external
input voltage sampling circuit electrically connected to the
microcontroller, and configured for sampling an external input voltage
from the probes; a battery voltage sampling circuit electrically
connected to the microcontroller, and configured for sampling a voltage
of the battery unit; and a voltage regulator circuit electrically
connected the microcontroller, wherein the microcontroller compares the
sampled external input voltage and the sampled voltage of the battery
unit, and controls the voltage regulator circuit to regulate the external
input voltage to be applicable to the battery unit based on the
comparison.

2. The multimeter of claim 1, wherein the voltage regulator circuit
comprises a voltage booster circuit and a voltage bleeder circuit in
parallel connection to the microcontroller, the voltage booster circuit
configured for boosting the external input voltage when the external
input voltage is too low to be applicable to the battery unit, and the
voltage bleeder circuit configured for bleeding the external input
voltage when the external input voltage is too high to be applicable to
the battery unit.

3. The multimeter of claim 1, wherein the charging system further
comprises a charging current monitoring circuit electrically connected to
the microcontroller and electrically coupled to the voltage regulator
circuit and the battery unit, and configured for monitoring magnitude of
the charging current and sending out a monitoring signal to the
microcontroller to adjust the charging current.

4. The multimeter of claim 3, wherein the charging system further
comprises a solid state relay driving circuit electrically connected to
the microcontroller and electrically coupled to the voltage regulator
circuit and the battery unit, the microcontroller controls the solid
state relay driving circuit to activate or stop the charging process.

5. The multimeter of claim 1, wherein the charging system further
comprises a charging mode selectable circuit comprising a first charging
mode circuit and a second charging mode circuit in parallel connection to
the microcontroller, the first charging mode circuit and the second
charging mode circuit each configured to send out a corresponding battery
type information to the microcontroller to activate the charging process.

6. The multimeter of claim 5, wherein the main body has two switchable
keys separately arranged on a surface of the main body, the switchable
keys corresponding to the respective first and second charging mode
circuits.

7. The multimeter of claim 5, wherein the battery type includes micro
pulse current rechargeable alkali batteries and continuous current
rechargeable batteries.

Description:

BACKGROUND

[0001] 1. Technical Field

[0002] The present disclosure relates to multimeters, and particularly to
a multimeter with a charging system.

[0003] 2. Description of Related Art

[0004] Typical multimeters use one time batteries or rechargeable
batteries as a source of power. When the batteries are exhausted, they
have to be changed or to be recharged, and the multimeters have to wait
the new power to go on work.

[0005] What is needed, therefore, is a multimeter, which can overcome the
above shortcomings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Many aspects of the present disclosure can be better understood
with reference to the following drawings. The components in the drawings
are not necessarily drawn to scale, the emphasis instead being placed
upon clearly illustrating the principles of the present disclosure.
Moreover, in the drawings, like reference numerals designate
corresponding parts throughout the several views.

[0007] FIG. 1 shows a multimeter including a main body, two probes
electrically connected to the main body, a battery unit, and a charging
system.

[0008] FIG. 2 is a working principle chart of the charging system of the
multimeter of FIG. 1.

[0009] FIGS. 3-5 are circuits of the charging system of multimeter of FIG.
1.

DETAILED DESCRIPTION

[0010] Embodiments of the present disclosure will now be described in
detail below and with reference to the drawings.

[0011] FIG. 1 shows a multimeter 100 including a main body 10, two probes
11 electrically connected to the main body 10, a battery unit 12, and a
charging system 20. The charging system 20 controls charging of the
battery unit 12 of the multimeter 100 through the probes 11.

[0012] The main body 10 has two switchable keys K1 and K2 separately
arranged on a surface of the main body 10. The battery unit 12 can have
micro pulse current rechargeable alkali battery or continuous current
rechargeable batteries, such as lithium ion battery. The keys K1
corresponds to the alkali battery, and the keys K2 corresponds to the
other rechargeable battery to switch on or off for charging the battery
unit 12.

[0013] The charging system 20 includes a microcontroller 21 with a
charging mode selectable circuit 22, an external input voltage sampling
circuit 23, a battery voltage sampling circuit 24, a voltage regulator
circuit 25, a charging current monitoring circuit 26 and a solid state
relay driving circuit 27 each controlled by a leading pin of the
microcontroller 21. In one embodiment, the microcontroller 21 can be
PIC16F73 of MIRCOCHIP Corporation.

[0014] The charging mode selectable circuit 22 is used to send out a
battery type signal, for example micro pulse current rechargeable alkali
battery type signal or continuous current rechargeable battery type
signal to the microcontroller 21, such that the microcontroller 21
activates an applicable charging mode for the battery. The charging mode
selectable circuit 22 includes a first charging mode circuit 221 and a
second charging mode circuit 222 in parallel connection. The first
charging mode circuit 221 includes a resistor R1 and a switch SW1 in
series connection, and two ends of the resistor R1 are electrically
connected to the switch SW and a power VCC, respectively. An electrical
contact D1 is arranged between the resistor R1 and the switch SW1. The
second charging mode circuit 222 includes a resistor R2 and a switch SW2
in series connection, and two ends of the resistor R2 are electrically
connected to the switch SW2 and the power VCC. An electrical contact D2
is arranged between the resistor R2 and the switch SW2. The first
charging mode circuit 221 is electrically connected to the
microcontroller 21 through the electrical contact D1, and the second
charging mode circuit 222 is electrically connected to the
microcontroller 21 through the electrical contact D2.

[0015] The switches SW1 and SW2 correspond to the keys K1 and K2 to switch
on or off the first and second charging mode circuits 221 and 222. In
application, the user selects one of the keys K1 and K2 to switch on the
corresponding one of the first and second charging mode circuits 221 and
222 according to the battery type. Then the charging mode selectable
circuit 22 sends out a battery type signal to the microcontroller 21,
such that the microcontroller 21 activates an applicable charging mode
for the battery. For example, when the alkali battery needs to be
charged, the user selects the key K1. Then, the microcontroller 21 may
use micro pulse currents to charge the battery, when a rechargeable
battery, such as lithium ion battery, needs to be charged, the user
selects the key K2, and then the microcontroller 21 may use a continuous
current to charge the battery.

[0016] It is understood that, if another charging mode is applicable for
the alkali battery, can be applied to the microcontroller 21.

[0017] The external input voltage sampling circuit 23 includes two
resistors R3 and R4 in series connection. One end of the resistor R3 is
electrically connected to both of the probes 11, the other end of the
resistor R3 is connected to the ground via the resistor R4. An electrical
contact D4 is arranged between the resistors R3 and R4 and the external
input voltage sampling circuit 23 is electrically connected to the
microcontroller 21 via the electrical contact D4.

[0018] The external input voltage sampling circuit 23 is used to sample
the input voltage from the probes 11, and send out the sample signal to
the microcontroller 21.

[0019] The battery voltage sampling circuit 24 includes two resistors R5
and R6 in a series connection. One end of the resistor R5 is connected to
the ground, the other end of the resistor R5 is electrically connected to
the positive terminal of the battery unit 12. An electrical contact D5 is
arranged between the resistors R5 and R6, and the battery voltage
sampling circuit 24 is electrically connected to microcontroller 21 via
the electrical contact D5.

[0020] The battery voltage sampling circuit 24 is used to sample the
voltage of the battery unit 12, and send out a voltage signal of the
battery unit 12 to the microcontroller 21.

[0021] The voltage regulator circuit 25 includes a voltage booster circuit
251 and a voltage bleeder circuit 252. The voltage booster circuit 251
and voltage bleeder circuit 252 each are electrically connected to the
microcontroller 21 and each are directly controlled by the
microcontroller 21. The voltage booster circuit 251 has an electrical
contact D6, and the voltage bleeder circuit 252 has an electrical contact
D7. The voltage booster circuit 251 and the voltage bleeder circuit 252
are electrically connected to the probes 11 via the electrical contacts
D6 and D7, respectively and each are configured to boost and bleed the
input voltage from the probes 11 based on the control of the
microcontroller 21.

[0022] The voltage regulator circuit 25 is electrically connected to the
positive of the battery unit 12 via two resistors R7 and R8, and is
configured to charge the battery unit 12. The microcontroller 21
determines to activate the voltage booster circuit 251 or the voltage
bleeder circuit 252 based on the voltage samples from the external input
voltage sampling circuit 23 and the battery voltage sampling circuit 24.

[0023] The charging current monitoring circuit 26 includes a first
operational amplifier circuit 261, a second operational amplifier circuit
262 and a third operational amplifier circuit 263. The first operational
amplifier circuit 261 is coupled to the voltage regulator circuit 25 and
the resistor R7 via an electrical contact D8 arranged at the forward
input end of the first operational amplifier circuit 261, and is
configured to sample the current output from the voltage regulator
circuit 25. The second operational amplifier circuit 262 is coupled to
the resistor R8 and the battery unit 12 via an electrical contact D9, and
is configured to sample the current input to the battery unit 12. An
output end of the first operational amplifier circuit 261 is electrically
connected to the inverse input end of the third operational amplifier
circuit 263 via a resistor R9, and an output end of second operational
electrical contact 262 is electrically connected to forward input end of
the third operational amplifier circuit 263 via a resistor R10. A
variable resistor R11 used as a potentiometer is arranged between an
inverse input end of the first operational amplifier circuit 261 and an
inverse input end of the second operational amplifier circuit 262. The
third operational amplifier circuit 263 is electrically connected to the
microcontroller 21 via an electrical contact D10 arranged at the output
end of the third operational amplifier circuit 263. The third operational
amplifier circuit 263 is configured to collect the current samples from
the first and second operational amplifier circuits 261 and 262, and send
out the current samples to the microcontroller 21, then the
microcontroller 21 obtains the magnitude of the charging current through
an analog-to-digital conversion.

[0024] The solid state relay driving circuit 27 includes electrical
contacts D11 and D12. The electrical contact D11 is arranged between and
coupled to the resistors R7 and R8, and the electrical contact D12 is
arranged between and coupled to the resistor R8 and the battery unit 12.
The microcontroller 21 activates or closes the charging function of the
multimeter 100 through the solid state relay driving circuit 27.

[0025] In application, user first selects one of the keys K1 and K2 based
on the type of the battery unit 12 to be charged, to switch on the
corresponding one of the switches SW1 and SW2, then external electrical
energy can go into the charging system 20 through the probes 11. The
external input voltage sampling circuit 23 first samples the input
voltage, then sends out the sample signal to the microcontroller 21, at
the same time the battery voltage sampling circuit 24 samples the voltage
of the battery unit 12 and sends out the sample signal to the
microcontroller 21. Then the microcontroller 21 activates the voltage
regulator circuit 25 to regulate the input voltage from the probes 11 to
be applicable for the charging mode, based on the sample signals from the
external input voltage sampling circuit 23 and the battery voltage
sampling circuit 24.

[0026] When the input voltage is greater than the voltage of the battery
unit 12, the microcontroller 21 controls the voltage bleeder circuit 252
to bleed the input voltage, such that the input voltage is lowered to be
applicable to the battery unit 12. When the input voltage is lower than
the voltage of the battery unit 12, the microcontroller 21 controls the
voltage booster circuit 251 to booster the input voltage, such that the
input voltage is boosted to be applicable to the battery unit 12.

[0027] During the charging of the battery unit 12, the charging current
monitoring circuit 26 samples and amplifies the output current from the
voltage regulator circuit 25, then sends out the sample signal to the
microcontroller 21. The microcontroller 21 then obtains the magnitude of
the charging current through an analog-to-digital conversion, and sends
out a control signal to the solid state relay driving circuit 27 based on
the magnitude of the charging current. In particular, when the charging
current is too high to be applicable for the battery, the microcontroller
21 sends out a stop signal to the solid state relay driving circuit 27 to
temporarily stop the charging. When a next period is found that the
charging current is applicable to the battery, then the microcontroller
21 sends out an activate signal to the solid state relay driving circuit
27 to activate the charging.

[0028] It is understood that once the input voltage and the resistor of
the charging system are applicable to the battery unit 12, the charging
current monitoring circuit 26 and the solid state relay driving circuit
27 can be omitted.

[0029] The multimeter 100 may include other powering circuits, for
example, powering circuits for powering the microcontroller 21.

[0030] The multimeter 100 of the present disclosure has a charging system
that charges the battery of the multimeter 100 using the external
electrical energy. That is, when the multimeter 100 is in use, the
battery unit 12 is being charged.

[0031] It is understood that the above-described embodiments are intended
to illustrate rather than limit the disclosure. Variations may be made to
the embodiments and methods without departing from the spirit of the
disclosure. Accordingly, it is appropriate that the appended claims be
construed broadly and in a manner consistent with the scope of the
disclosure.